1. Field of the Invention
This invention relates to the measurement of sediment concentration, sediment velocity, and the resulting sediment transport, and to the measurement of turbulent stresses and dissipation in the ocean, particularly in estuaries and the nearshore coastal boundary where sediment transport affects a wide range of human activity. More precisely this invention relates to field measurements of sediment transport in rivers, the near-shore coastal region, and coastal shelf regions, and to the concurrent measurement of turbulent processes which control sediment transport. Specifically the devices operate by noninvasively measuring both suspended sediment concentration and three component velocity vectors in the same, small volume, allowing their product, the three component sediment mass flux, to be directly estimated in the water column above the sediment bed, while resolving turbulent velocity scales. Furthermore, selection of the transmitted frequency allows the acoustic backscatter response to be matched to the size distribution of sandy sediments while rejecting scattering from fine muds which typically compromise optical backscatter estimates of sand transport.
2. Description of the Related Art
Previous sediment flux measurements have used optical backscatter sensors (for example, Huntley, D. A. and D. M. Hanes, 1987, "Direct Measurement of suspended sediment transport", Coastal sediments, 87. pp 723-737), profiling acoustic backscatter devices, (for example Hanes, D. M., C. E. Vincent, D. A. Huntley and T. L. Clarke, 1988, "Acoustic measurements of suspended sand concentration in the C.sup.2 S.sup.2 experiment at Stanhope Lane, Price Edward Island", Marine Geology, 81, pp 185-196), or pumped sampling systems (Thornton, E. B., and W. D. Morris, 1977, "Suspended sediments measured within the surf zone", Coastal Sediments 87, pp 655-668), to estimate the mass concentration at some level above the sediment bed. An in situ current meter (usually which measures only 2 of the 3 velocity components, and perturbs the flow being measured) is located near the mass sampling point. Mass flux, at scales much larger than the separation of the mass and current measurement locations (typically 10 to 50 cm), is estimated as the product of the mass concentration and the nearby measured velocity components. This large scale inherent in these separated mass and velocity measurements prevents sediment flux measurement at small scales found within or near the oscillatory, wave forced, boundary layer above the sediment bed, where much of the sediment transport occurs on coastal beaches. Studies using a Laser Doppler Velocimeter (Agrawal, Y., J. H. Trowbridge, H. C. Pottsmith and J. Oltman-Shay, "Velocity, concentration and flux of sediments in a coastal bottom boundary layer with a laser doppler velocimeter", Oceans 93, Proc. IEEE Conf. Ocean Engrg., Victoria, B.C., Canada) have improved these methods by measuring two components of velocity and relative sediment concentration at a single point.
Acoustic doppler measurements of fluid flow using doppler acoustic techniques are described in a range of patents using incoherent doppler sampling (for example David G. Shave, "Shipboard Apparatus for Measuring Ocean Currents," U.S. Pat. No. 4,138,657 (Feb. 6, 1979), and Blair H. Brumley, et al., "Broadband Acoustic Doppler Current Profiler," U.S. Pat. No. 5,615,173 (Mar. 25, 1997)). The pulsed coherent acoustic doppler method used in the present invention is discussed in Katakura, (Kageyoshi Katakura, et al., "Ultrasonic Velocity Meter," U.S. Pat. No. 4,751,847 (Jun. 21, 1988)), which describes the method of estimating a doppler velocity from a coherently sampled, complex demodulated acoustic signals. Similar signal sampling and signal processing techniques for pulsed coherent systems are widely published in the atmospheric radar literature, and are summarized in Doviak, J. and D. S. Zmic, 1984, "Doppler Radar and Weather Observations", Academic Press. A patent issued to David M. Farmer and R. Del Hudson, "Method and Apparatus for Simulating Phase Coherent Signal Reflections in Media Containing Randomly Distributed Targets," (U.S. Pat. No. 4,872,146 (Oct. 3, 1989)) discuss more broadly the theory of coherently sampling sparse, weak scatterers in a fluid to estimate the population count of discrete scatterers such as fish. The current invention broadly uses the underlying principles of coherent sampling to estimate backscatter strength, but is not concerned with finding the population count of discrete scatterers, but rather finding a concentration of scatterers with a broader, typically lognormal size distribution. This invention also describes an in situ calibration method which overcomes several limitations of acoustic models of ultrasonic backscatter from sandy sediments (see for example Sheng, J., and A. E. Hay, 1988, "An examination of the spherical scatterer approximation in aqueous suspensions of sand," J. Acoust. Soc. Am., 83, 598-610).